Lubricants in commercial use today are prepared from a variety of natural and synthetic base stocks admixed with various additive packages and solvents depending upon their intended application. The base stocks can include, e.g., Groups I, II and III mineral oils, gas-to-liquid base oils (GTL), Group IV polyalpha-olefins (PAO) including but not limited to PAOs made by using metallocene catalysts (mPAOs), Group V alkylated aromatics (AA) which include but are not limited to alkylated naphthalenes (ANs), silicone oils, phosphate esters, diesters, polyol esters, and the like.
Manufacturers and users of lubricating compositions desire to improve performance by extending oil drain life of the lubricating composition. Extended drain life is a highly desirable marketing feature of lubricating compositions, especially Group IV/Group V lubricating compositions.
Between machine elements, a thin film of lubricant wedges itself between the would-be contacting surfaces, thereby inhibiting metal-to-metal contact. In electrohydrodynamic lubrication (EHL) conditions, the contact pressures are so high that formation of such film wedge is extremely difficult.
Component in a formulation, typically with higher viscosity, that can provide good EHL film thickness is sought after for that will benefit the lubricant performance in EHL condition. However, the EHL film thickness of a family of molecular structure, typically, increases with increased molecular size, i.e. with increased viscosity at the given operating temperature. But in a given formulation classification, commonly with required final viscosity range, the amount of high viscosity material used must decrease with its viscosity increase following the blending rules. In addition, increase of the viscosity of the high viscosity component and/or increase the high viscosity component treat rate often means that the low temperature performance of the formulation must carry a debit. The formulators must find a balance considering these effects when evaluating a high viscosity component.
Moreover, it is desired that lubricant compositions provide sufficient protection of lubricated surfaces, particularly wear protection for gear box oils and automotive transmission oils.